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graphene by laser chemical vapor deposition with high electrochemical performance
Journal of Power Sources 444 (2019) 227308
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Nanoforest of 3C–SiC/graphene by laser chemical vapor deposition with high electrochemical performance Qingyun Sun a, Rong Tu a, Qingfang Xu a, Chitengfei Zhang a, Jun Li b, Hitoshi Ohmori c, Marina Kosinova d, Bikramjit Basu e, Jiasheng Yan f, Shusen Li f, Takashi Goto a, Lianmeng Zhang a, Song Zhang a, * a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, China National Key Laboratory for Shock Wave and Detonation Physics, Institute of Fluid Physics, P.O. Box 919-102, Minyan, 621900, China Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan d Nikolaev Institute of Inorganic Chemistry, Russian Academy of Sciences Siberian Branch, 3 Acad. Lavrerntiev Pr. Novosibirsk, 630090, Russia e Centre for BioSystems Science and Engineering, Indian Institute of Science, Bangalore, 560012, Karnataka, India f Tech Semiconductors, LTD, No.162 Shengli Street, Xiangyan, 441021, China b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Nanoforest of 3C–SiC/graphene was deposited by laser chemical vapor deposition. � The material shows capacitance of 8.533 mF/cm2 at a current density of 20 μA/cm2. � Improved capacitances per area and excellent cycle stability were observed.
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoforest 3C–SiC films Graphene LCVD Electrochemical performance
Nanoforests of 3C–SiC/graphene films are prepared by laser chemical vapor deposition (LCVD). The effectiveness of nanoforest-like 3C–SiC/graphene film as electrode materials for supercapacitors has been investigated by cyclic voltammetry and galvanostatic charge-discharge tests in 0.5 M H2SO4 solution. The specific capacitance is 8.533 mF/cm2 at a current density of 20 μA/cm2, which is 15 times higher than of previous reports of composited 3C–SiC/graphene films. The electrode exhibits good rate capability and cycling stability with 90.5% capacitance retention after 10000 cycles. The nanoforest-like 3C–SiC/graphene thick film shows a 3D porous structure with exposed graphene conductive network contributing to the greatly enhanced electrochemical performance in an environmentally aqueous electrolyte. These nanoforest-like 3C–SiC/graphene films can be promising for elec trochemical energy storage applications.
* Corresponding author. E-mail address: [email protected] (S. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227308 Received 19 July 2019; Received in revised form 13 September 2019; Accepted 13 October 2019 Available online 22 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
electrode has been investigated in detail and further compared with previous researches.
The ever-increasing pollution concerns and energy demands have kindled worldwide research interest in developing more efficient and clean alternative energy storage devices [1]. Supercapacitors, especially electrical double-layer capacitors (EDLCs) have attracted more and more attention due to their significant advantages, such as high power density, long cycle-life, and safe operation with high rate charge and discharge [2–4]. For planer micro-supercapacitor, both the electrode properties and large-scale manufacturability need to be considered. Carbon-based materials, including activated carbon [5,6], carbon nanotubes [7,8], carbon nanowires [9] and graphene [10] have been widely investigated as electrode materials in supercapacitor. Among them, graphene has attracted much attention due to higher specific surface area and high conductivity [11–13]. However, in most studies, graphene capacitors electrodes are fabricated by organic binders from powders. The graphene layers intend to restack, leading to reduced accessible surface area with electrolyte [14,15]. Recently, 3C–SiC materials have been demonstrated recently as a promising semiconductor material for planer micro-supercapacitors due to the excellent chemical stability, good mechanical robustness, wide band gap and high electron mobility. Especially, 3C–SiC can be depos ited by chemical vapor deposition below Si melting point (1683 K), which can be compatible with the current Si based semiconductor in dustry [16]. Several efforts focused on 3C–SiC planer micro-supercapacitor electrodes (on Si chips) have already been re ported in the literature [2,17–23]. Yang et al. have demonstrated the potential of nanocrystalline 3C–SiC films to be used as supercapacitor electrodes [24,25]. Although it has a high surface efficiency, due to a low surface area of nanocrystalline film structure, its specific capaci tance was only 0.073 mF/cm2. Alper et al. have fabricated 3C–SiC nanowires on Si substrate by low pressure CVD method. The electrodes achieved capacitance values to 0.240 mF/cm2 [17]. However, 3C–SiC materials exhibit low electrical conductivity which limits its perfor mance as EDLCs electrodes [14]. The performance of EDLCs is greatly depended on the specific surface area as well as the conductivity and stability of their electrode materials. The excellent electrical conductivity of the electrode materials can significantly reduce the internal resistance of an electrode, by forming a conductive network, and facilitate the application of electrostatic charges, which favor the accumulation of electric double layers [2,26]. The addition of graphene as filler was proved to significantly enhance the electrical conductivity of 3C–SiC materials [27,28]. However, a few studies were reported to use the 3C–SiC/graphene as electrode materials for supercapacitors. Currently, Heuser at el [14] synthesized 3C–SiC/graphene hybrid nanolaminate and investigated the perfor mance as supercapacitors electrode. The 3C–SiC/graphene electrode exhibited a high specific capacitance of 0.550 mF/cm2 which are significantly higher than uniform 3C–SiC films [24]. The layer number of graphene was about 15 between 3C–SiC layers. However, the elec trical activity and wettability of graphene will dramatically reduce by numbers of stacking layers, which will limit its performance in super capacitors [17,29]. Furthermore, the distribution and exposed area of graphene surface (active area) plays important role on supercapacitors performance. Hence, there is a need for controlling 3C–SiC/graphene film morphology with higher surface area directly on Si chips that can keep stable excellent capacitive behavior for long time circle and adapt to microfabrication process in manufacturing of planar micro-supercapacitor devices. Laser chemical vapor deposition (LCVD) was developed by our research group which could control different structure and morphology of films [16,30]. By controlling the morphology of porous and conduc tive electrode materials is an effective way to provide them with high surface areas and efficient paths for electron diffusion [2]. In this study, we have carried out 3C–SiC/graphene nanoforest films directly on Si substrate. The performance of 3C–SiC/graphene nanoforest as EDLCs
2. Experimental 2.1. Preparation of 3C–SiC/graphene films A self-made horizontal cold-wall laser chemical vapor deposition (LCVD) apparatus was employed to deposit nanoforest-like 3C–SiC films on Si (110) single crystalline substrate (15 � 10 � 0.5 mm, Hefei Kejing Inc., China), with hexamethyldisilane (HMDS, Tokyo Chemical Inc., Japan) as a single source. H2 (purity 99.999%, Wuhan Minghui Inc., China) and Ar (purity 99.999%, Wuhan Minghui Inc., China) was used as dilution gas and carrier gas, respectively. The laser (HK1064–500FC, wavelength: 1064 nm, Beijing ZK laser, China) was operated in the continuous mode. The deposition temperature (Tdep) ranged from 1473 to 1623 K and was measured by an infrared pyrometer (CT laser, 673–1873 K, Optris, Germany). The total pressure and deposition time was at 400–1200 Pa and 20 min, respectively. The source HMDS was evaporated from a stock tank at 300 K and was carried into the depo sition chamber with Ar and H2 gas. The flow rate of diluted H2 gas and carrier Ar gas was set as 500 and 25 standard cubic centimeter per minute (sccm), respectively. Fig. 1 shows the schematic of the LCVD apparatus and the synthesis process of nanoforest of 3C–SiC/graphene. In the LCVD process, the morphology of film can be designed by adjusting the deposition parameters to control the nucleation rate and growth rate of 3C–SiC. In order to obtain the higher specific surface, the nanoforest-like 3C–SiC overlapped by nano-whiskers was deposited at 1523 K and 400 Pa. The graphene and 3C–SiC growth with one step. In the deposition process, the laser and the H2 has an etching effect on SiC whiskers, the Si atom on the edge will be etched by H2 forming Si atom vacancy. Then the rest C atoms combined each other by C–C band to grow graphene epitaxially on SiC whisker edge. 2.2. Structure characterization The surface and cross-section morphology of deposited films were observed by field emission scanning electron microscopy (FESEM; Quanta-250, FEI, Houston, USA; 20 kV). Raman spectrum (Invia, Renishaw, UK; 514.5 nm) measurement was used to evaluate the composition and phase of deposited films. Scan Transmission electron microscopy (STEM, TALOS F200S, USA) was used to observe the 3D morphology of the nanoforest-like 3C–SiC/graphene film obviously. The Energy Dispersion Spectroscopy (EDS) on STEM was used to analysis the element and distribution of nanoforest-like 3C–SiC/graphene film. Transmission electron microscopy (TEM, JEOL JEM-2100, Tokyo, Japan; 200 kV) was used to observe the micro-structure and phase of the deposited films. 2.3. Electrochemical and wettability testing Capacitive performance of the films was investigated using an elec trochemical workstation (CHI 660e) in a three electrodes system. The electrochemical properties were measured by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) methods. A Pt foil was used as counter electrode, and a Saturated Calomel Electrode (SCE) was used as the reference electrode. All the measurements were performed in an aqueous electrolyte solution of 0.5 M H2SO4 at room temperature. Prior to acquisition, samples were cycled 20 times to ensure removal of any adsorbed contaminants. The wettability of 0.5 M H2SO4 electrolyte solution on surface of films was conducted on contact angle measurement system. 3. Results and discussion Fig. 2 shows the typical surface and cross-section SEM micrograph of 2
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Fig. 1. The schematic of the LCVD apparatus and the synthesis process of nanoforest of 3C-SiC/graphene film
different morphologies of deposited 3C–SiC films. Fig. 2 (a) presents the surface image of nanoforest-like 3C–SiC film with magnified surface image of single whisker showed in the right-corner. A high density of 3C–SiC nano-whiskers with average diameters about 60 nm and length about 500 nm were grown on the surface. The whiskers looked like needles with sharp tips. Fig. 2 (d) shows the cross-section image of the nanoforest-like 3C–SiC film, and the magnified image of the crosssection growth was shown in Fig. 2 (g). The numerous SiC nanowhiskers like the branches grew around an imaging trunk with the top oblique upward, appearing like “nanoforest” standing vertically on the Si substrate. The surface and cross-section images of the flower-like and pyramid-like films were shown in Figs. 2 (b), (e), (h) and Figs. 2 (c), (f), (i), respectively. The typical grain morphology of the flower-like and pyramid-like film was shown in the right-corner of Figs. 2 (b) and (c), respectively. Some intervals existed between grains of nanoforest-like and flower-like films, as shown in Fig. 2 (g) and (h), while the crosssection image of pyramid-like film was dense, as shown in Fig. 2 (i). The simulated diagrams of three typical morphologies (nanoforest-like, flower-like and pyramid-like) were shown in corresponding areas in Fig. 2 The thickness of nanoforest-like, flower-like and pyramid-like 3C–SiC film was about 20.7 μm, 13.8 μm, 12.1 μm in 20 min deposi tion process by LCVD, respectively. The effect of Tdep and Ptot on the morphologies of films prepared by LCVD was shown in Fig. S1. The nanoforest-like films were obtained at low Tdep. With increasing Tdep, the crystalline of films increased, the grains of film grew densely. At high Tdep and low Ptot, the pyramid-like films were obtained. Whereas at high Tdep and high Ptot, the flower-like films were deposited, this is probably due to a relative higher concen tration of HMDS at higher Ptot [31]. The selected samples represent three typical morphology as marked A, B and C will been compared in later discussions.
Fig. 3 shows the Raman spectra of selected films (A, B, C) at different Tdep and Ptot. The Raman shift from 550 to 1200 cm 1 was conducted in the Raman test. The peak at ~1345 cm 1 and ~1598 cm 1 was related to carbon D and G band, respectively. The observation of carbon 2D band at ~2675 cm 1 indicated the presences of multilayered graphene [32]. The characteristic Raman modes of 3C–SiC are transverse optical (TO) phonon and longitudinal optical (LO) phonon, corresponding to 796 and 972 cm 1, respectively [24]. In the case of sample A (nano forest-like films) deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa and sample B (flower-like films) deposited at Tdep ¼ 1573 K and Ptot ¼ 800 Pa, the typical features for 3C–SiC as well as for graphite and graphene were observed, confirming the formation of 3C–SiC/Graphene composited films [33]. The D’ peak at 2950 cm 1 indicated the defects in graphene [14]. The TO peak was much higher than LO peak which was probably caused by stacking defects in the 3C–SiC film [34,35]. While in the case of sample C (pyramid-like films) deposited at Tdep ¼ 1623 K and Ptot ¼ 400 Pa, TO and LO peaks without characteristic peak for carbon phase were observed, indicating pure phase of 3C–SiC deposits. Fig. 4 shows the TEM observations and atom simulated structure of nanoforest-like 3C–SiC/graphene film deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. The TEM image of cross-section morphologies for nanoforest-like 3C–SiC/graphene film was shown in Fig. 4(a). In the film part, the black area presented the overlapped 3C–SiC nano-whiskers, the pores between whiskers can been vaguely seen from the penetrated light. The selected area electron diffraction (SAED) analysis for the nanoforest-like 3C–SiC film in white square of Fig. 4(a) showed a pattern of three rings (Fig. 4(b)), indicating the film is polycrystalline growth. The polycrystalline rings are indexed to (111), (220), (311) plane of 3C–SiC, respectively. Fig. 4(c) shows the cross-section morphology of the individual 3C–SiC nano-whiskers. Fig. 4(d) shows high-resolution 3
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Fig. 2. Surface and cross-section FESEM images of different morphologies for deposited films. (a), (b), (g): Nanoforest-like film deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. (b), (e), (h): Flower-like film deposited at Tdep ¼ 1573 K and Ptot ¼ 800 Pa. (c), (f), (i): Pyramid-like film deposited at Tdep ¼ 1623 K and Ptot ¼ 400 Pa.
transmission electron microscopy (HRTEM) for the edge of 3C–SiC sin gle whisker in the black square of Fig. 4(c). The lattice fringe spacing of 0.252 nm as heighted is related (111) and (11–1) plane (with a 71� angle) of 3C–SiC, meaning that the 3C–SiC whiskers grew in <111> orientation. The lattice fringe spacing of 0.334 nm as heighted in Fig. 4 (d) is related graphene growth. The growth mechanism of 3C–SiC/gra phene nanoforest was discussed by atomic configuration, as shown in Fig. 4(e). Fig. 4(d) was observed from [ 1-12] axis, while Fig. 4(e) was simulated from Refs. [1–10] axis, since the generated twins of 3C–SiC only can be observed from Refs. [1–10] axis. The twinning boundary (TB) was on (111) plane, as simulated in Fig. 4(e). Under the irradiation of laser in H2 atmosphere, the Si atom on the edge of waved (11–1) plane will be etched by H2, forming Si atom vacancy [36]. Then the rest C atoms combined each other by C–C band to grow graphene epitaxially on the (11–1) plane of SiC whisker edge, as shown in Fig. 4(e). The growth direction of SiC whisker and graphene are vertical. The graphene grew on the SiC (11–1) plane, because a waved (11–1) plane formed by TB growth has a relative lower surface energy. Fig. 5 shows the STEM image and element distribution analysis for nanoforest-like 3C–SiC/graphene film deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. The STEM images of cross-section morphologies for nanoforest-like 3C–SiC film were shown in Fig. 5 (a) and (b). In the film
part, the bright area presented the overlapped 3C–SiC nano-whiskers, the pores between whiskers can be seen from the penetrated dark area obviously. The magnified STEM image of the cross-section nanoforestlike 3C–SiC/graphene film was shown in Fig. 5 (b). The growth direction of all the whiskers was random by crisscross overlapped each other, forming a 3D pores network structure. Compared with TEM image, the STEM image is more efficient to characterize the pores network struc ture of nanoforest-like 3C–SiC film. The element analysis of nanoforestlike 3C–SiC/graphene film was characterized by energy dispersion spectroscopy (EDS), as shown in Fig. 5 (c)–(f). Only signals of C and Si are observed in the EDS analysis. The topography consisted of Si element (Fig. 5 (c)) was corresponding to the whole topography of nanoforestlike 3C–SiC/graphene film (Fig. 5 (a)). Some additional C element related to graphene was distributed in the pores area between 3C–SiC whiskers, as shown in Fig. 5 (e). This EDS map also indicated that the graphene was formed on the edge of 3C–SiC whiskers, corresponding the TEM results (Fig. 4(d)). The TEM observations for pyramid-like 3C–SiC film deposited at Tdep ¼ 1623 K and Ptot ¼ 400 Pa was shown in Fig. S2. The TEM image of cross-section morphologies for pyramid-like 3C–SiC film was shown in Fig. S2(a). The grains grew with pyramid tips, related to SEM image (Fig. 2(f)). A polycrystalline diffraction pattern was also observed in the 4
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selected area electron diffraction (SAED) analysis for the pyramid-like 3C–SiC film (Fig. S2(b)). While some strong diffraction spots including diffraction spot from (111) plane of 3C–SiC in growth direction were obvious, indicating strong <111> 3C–SiC orientation. Fig. S2(c) shows the cross-section morphology of 3C–SiC grains. The grains were larger than 600 nm. Fig. S2(d) shows high-resolution transmission electron microscopy (HRTEM) for the boundary of 3C–SiC grains in the square of Fig. S2(c). There was no graphene found on the boundary of 3C–SiC grains as well as other area of films. Since the CV and GCD measurements were conducted in aqueous electrolyte, the wettability of electrode has a direct influence on the capacitive performance. Fig. S3 shows the wettability of different mor phologies of deposited films. The contact angle is dependent tightly on the surface morphology of the film, especially on the space between the grains on surface [37]. The surface of nanoforest-like 3C–SiC/graphene film had a good hydrophilia with a contact angle of 48� . While the flower-like 3C–SiC/graphene film and pyramid-like 3C–SiC film showed a hydrophobic with contact angle of 105� and 132� , respectively. A small contact angle would result in a better capacitive performance, which will improve the affinity of the electrode towards the electrolyte [38]. The electrochemical properties of the films were characterized using cyclic voltammetry. The double layer capacitances of the films were first evaluated in the potential window of 0.5–0.9 V with scan rate from 10 to 50 mV/s in 0.5 M H2SO4. Fig. 6 (a) show the CV curves at a scan rate of 50 mV/s. For a potential window of 0.4 V, the shape of the CV curves of
Fig. 3. Raman spectra of the films deposited at different Tdep and Ptot. (A): Nanoforest-like films deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. (B): Flower-like films deposited at Tdep ¼ 1573 K and Ptot ¼ 800 Pa. (C): Pyramidlike films deposited at Tdep ¼ 1623 K and Ptot ¼ 400 Pa.
Fig. 4. TEM observations ((a)-(d)) and the atomic configuration (e) of the nanoforest-like 3C-SiC/graphene films deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. 5
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Fig. 5. STEM morphology observation ((a), (b)) and EDS analysis ((c)-(e)) of the nanoforest-like 3C-SiC/graphene film deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa.
three samples were symmetrical and rectangular, indicating an ideal capacitive behavior [39]. The current density of the nanoforest-like and flower-like 3C–SiC/graphene films were much higher than pyramid-like 3C–SiC films. Its double layer capacitances (Cdl, mF/cm2) were calcu lated by using the equation of C ¼ j/ν (j is the background current density in mF/cm2 and ν is the scan rate in V/s) [24]. The average specific capacitances (Cdl) of three samples were calculated at scan rate from 10 to 50 mV/s as shown in Fig. 6 (b). Note that, the plane surface area of the films was taken into consideration to calculate the current density and Cdl, the inside area of film was not considered. The average specific capacitances of pyramid-like 3C–SiC films was only 0.02 mF/cm2. The value is close to epitaxial 3C–SiC films (0.006 mF/cm2) [24]. While for nanoforest-like and flower-like 3C–SiC/graphene films, the Cdl was 7.350 mF/cm2, 2.830 mF/cm2, respectively, which are more than 140–360 times higher compared to that of pyramid-like 3C–SiC film. The significant difference of the capacitance of three morphologies (sample A, B, C) comes from there variant specific surface area and conductivity, since the capacitance of a capacitor electrode is proportional to its active surface area and con ductivity [40]. The graphene formed on the boundary of SiC grains strongly improve the conductivity of deposited 3C–SiC/graphene films in comparison with pure 3C–SiC films [31]. In addition, compared with flower-like 3C–SiC/graphene films, the better hydrophily and porous network structure of nanoforest-like 3C–SiC/graphene film will contribute to the high electrochemical reaction surface in electrolyte, resulting in higher specific capacitance [14]. The nanoforest-like 3C–SiC/graphene films are expected to be a promising candidate for supercapacitor applications, since it shows a relatively higher double layer capacitance. The capacitive behavior of the nanoforest-like 3C–SiC/graphene films with a potential window from 0.2 to 1.0 V has been investigated. Fig. 6 (c) show the CV curves of the nanoforest-like 3C–SiC/graphene film in 0.5 M H2SO4 at scan rate from 10 to 100 mV s. The CV curves for all the scan rates showed quasi-
rectangular shape obviously in the potential range scanned. The CV curve was symmetric, as expected for a typical characteristic of a capacitor. The charge-discharge behaviors at different current density from 20 to 200 μA/cm2 were shown in Fig. 6 (d). For all current densities tested, the segments of anodic charging curves were almost symmetric to the cathodic discharge curves, indicating the perfect reversibility properties of its capacitor behavior. The average specific capacitance was thus calculated from the CV curves (C ¼ j/ν, j is the background current density in mF/cm2 and ν is the scan rate in V/s) and the chargedischarge curves (C ¼ jΔt/ΔV, j is the background current density in mF/ cm2, ΔV is the potential range in V and Δt is the discharge time in s), as shown in Fig. 6(e) and Fig. 6 (f), respectively. In the case of charge current density, the specific capacitance decreased by 36% (from 8.533 to 5.425 mF/cm2 with a charge current density from 20 to 200 μA/cm2). The same as other capacitors, its specific capacitance decreased with an increase of the scan rate and current density. Nevertheless, only 20% reduction of its capacitance was observed even with 20-fold increase of the scan rate (from 8.480 to 6.680 mF/cm2 with a scan rate from 10 to 200 mV/s). With 20-fold increase of the scan rate (with a scan rate from 10 to 200 mV/s), the nanocrystalline 3C–SiC films on Si substrate showed a 34% reduction of its capacitance [24]. While the SiC nano wires on carbon fabric showed a 50% reduction of its capacitance [21]. Therefore, good rate capability has been achieved on the nanoforest-like 3C–SiC/graphene films. Fig. 7 shows the Nyquist spectra of the nanoforest-like 3C–SiC/gra phene films measured from 10 kHz to 0.01 Hz. From the Nyquist plot, solution resistance can be got from the Z’ axis intercept at the high frequency end. Normally, the spectrum shows a semicircle which is represented by a parallel combination of interfacial capacitance and resistance. The width of semicircle plotted was indicative of the chargetransfer resistance (Rct) in the electrode. In the mid-frequency range, the spectrum shows the Warburg diffusion element, an important factor to affect the performance of supercapacitors. At very low frequencies, a 6
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Fig. 6. (a) CV curves for different mor phologies of films at 50 mV/s within a po tential window of 0.4 V and (b) linear fitting of the background current density (j) vs scan rate; (c) CV curves within a potential win dow of 0.8 V for nanoforest-like 3C-SiC/gra phene films at different scan rates; (d) charge-discharge curves for nanoforest-like 3C-SiC/graphene films with different cur rent densities; specific capacitances of the nanoforest-like 3C-SiC/graphene film vs scan rate (e) and charge current density (f).
vertical line can be observed which is caused by the accumulation of ions at the bottom of the pores of the electrode [2,23]. The Nyquist plot is similar to the previous report of composited 3C–SiC/graphene films [14], and the Rct of nanoforest-like 3C–SiC/graphene film was calcu lated from the Nyquist spectra with 126.5 Ω. The cycle durability is one of the most electrochemical properties of supercapacitors. Typical Nyquist plots the nanoforest-like 3C–SiC/graphene electrode after 1st and 10000th cycles are shown in Fig. 7. The arc increment from the 1st to the 10000th cycles is not obvious, indicating the nanostructures are well maintained and preserved overall with little structural deformation after 10000 cycles. Furthermore, the increase of the Warburg resistance after 10000 cycles can be attributed to the loss of adhesion of some active materials blocking the diffusion path ways of electron during the charge–discharge process. The long-time cyclic specific capacitance variation of nanoforest-like
3C–SiC/graphene films by galvanostatic charge-discharge (with poten tial from 0.5 to 0.9 V) at a current density of 100 μA/cm2 for 10000 cycles was presented in Fig. 8 (a). A capacitance loss about 3% is ob tained after about 200 circles. After 5000 cycles, the specific capacitance of nanoforest-like 3C–SiC/graphene electrode changes from 6.350 mF/ cm2 of first cycle to 5.950 mF/cm2, which keeps 93.7% capacitance retention. Whereas the capacitance of 3C–SiC/graphene hybrid nano laminate remained about 86% after 5000 cycles [14]. Similar stability of 87–90% has been reported on SiC/graphene nanoparticle after 6000 cycles [41]. After 10000 cycles, the capacitance was 5.750 mF/cm2, which keeps 90.5% capacitance retention after 10000 cycles. The 10 cycles of charge-discharge curves of the first and after 10000 cycles were presented in Fig. 8 (a). Therefore, good cycle stability has been achieved on the nanoforest-like 3C–SiC/graphene films. The highest specific capacitance of nanoforest-like 3C–SiC/graphene 7
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illustration of 3C–SiC/graphene nanoforest films with stable framework and continuous electron pathways. The outstanding electrochemical properties of nanoforest-like 3C–SiC/graphene film would attribute to three factors. Firstly, the composite of 3C–SiC/graphene electrode can combine the advantage of the excellent chemical stability, good mechanical robustness from 3C–SiC and high conductivity, good electrochemical activity from gra phene. The addition of graphene in 3C–SiC will dramatically improve the conductivity that attribute to significant increased capacitance. Secondly, the nanoforest-like 3C–SiC/graphene film showed a 3D porous network structure with good hydrophily will contribute to the high electrochemical reaction surface in aqueous electrolyte. Both the 3C–SiC whiskers and graphene layers are in nanoscale resulting high specific surface. Third, as schematically demonstrated in Fig. 8 (b), the graphene grown epitaxially on the edge of single 3C–SiC nano-whiskers in the forest structure constituted an electrically conductive network. The graphene in 3D pores provide efficient pathways for electron transport. The nanoforest-like 3C–SiC/graphene film directly grew on Si substrate avoiding the use of binders. Intervals were existed between 3C–SiC/graphene nano-whiskers, and substantially avoided the stacking issues and the “dead volume” of graphene in the electrode. Compared with the previous reports of composited 3C–SiC/graphene films, the layers of graphene were more than 10 layers, Since the electrical activity of graphene will dramatically reduce by numbers of stacking layers, which will limit its performance in supercapacitors. The graphene sur face in present electrode with less layers (2–3 layers) are highly exposed inside of 3C–SiC/graphene nanoforest. Therefore, the high electrical conductivity and high specific capacitance of graphene, together with good cycle ability of 3C–SiC and 3D pores forming in 3C–SiC/graphene nanoforest attributed to significantly improved electrochemical perfor mance due to the synergistic effects.
Fig. 7. Nyquist plots of the nanoforest-like 3C-SiC/graphene electrode. Inset magnifies the spectra in the high-frequency range with first and after 10000 circles.
film reached to 8.533 mF/cm2 at a current density of 20 μA/cm2. The highest volumetric capacitance was 4.1 F/cm3 calculated from the CV curves at 10 mV/s. Table 1 compares the double-layer capacitance of SiC film materials used for double-layer capacitor applications. The specific capacitance of nanoforest-like 3C–SiC/graphene film in present study was much higher than 3C–SiC films on Si substrate [17,24,42]. While this value enhanced 15 times higher than of previous reports of composited 3C–SiC/graphene films [14]. Fig. 8 (b) shows the schematic
Fig. 8. (a) Cycling performance of 3C-SiC/graphene nanoforest films of charge-discharge at 100 A/cm2. (b) Schematic illustration of 3C-SiC/graphene nanoforest films with stable framework and continuous electron pathways. Table 1 Specific capacitance of the SiC film materials used for planar double-layer capacitor applications. Electrode material
Substrate
Electrolyte
Double-layer capacitance (mF/cm2)
Volumetric capacitance (F/cm3)
Year
Refs
3C–SiC/graphene nanoforest films 3C–SiC/graphene hybrid nanolaminate Nanocrystalline 3C–SiC films Epitaxial 3C–SiC film SiC nanowires films N-doped Polycrystalline 3C–SiC film
Si Si Si Si Si Si
0.5 M H2SO4 NaSO4 0.1 M H2SO4 0.1 M H2SO4 3.5 M HCl 1 M H2SO4
5.425–8.533 0.458–0.550 0.040–0.073 0.005–0.006 0.135–0.240 0.743
4.1 1.1 1.0 0.2 0.4 2.9
2019 2018 2015 2015 2013 2011
This work [14] [24] [24] [17] [42]
8
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4. Conclusion
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In summary, LCVD method was used to prepare nanoforest-like 3C–SiC/graphene thick film using hexamethyldisilane (HMDS) in H2 atmosphere. The electrochemical properties of nanoforest-like 3C–SiC thick film as electrode in 0.5 M H2SO4 solution for supercapacitors were studied. The highest specific capacitance was 8.533 mF/cm2, which are 15 times higher than previous reports. The 3C–SiC/graphene nanoforest electrode shows good rate capability and cycling stability with 90.5% capacitance retention after 10000 cycles. The improved electrochemical properties of the electrodes are attributed to synergistic effects of high electrical conductivity and high specific capacitance of graphene, together with good cycle ability of 3C–SiC and 3D pores forming in 3C–SiC/graphene nanoforest. Therefore, 3C–SiC/graphene nanoforest films are potential electrode materials for big capacitance and highly stable energy storage devices. Acknowledgments This work was supported by the Joint Fund of the Ministry of Edu cation for Pre-research of Equipment, China (6141A02022257), the Science Challenge Project, China (No. TZ2016001), the National Natural Science Foundation of China, China (Nos. 51861145306, 51872212 and 51972244), and the 111 Project, China (B13035). It was also supported by the International Science & Technology Cooperation Program of China, China (2018YFE0103600, 2014DFA53090), the Technological Innovation of Hubei Province, China (2019AAA030), the Fundamental Research Funds for the Central Universities, China (WUT: 2018YS003, 2018YS016, 2019III030, 2019III028), and the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, China (WUT, Grant No. 2019-KF-12). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227308. References [1] K. Xie, J. Li, Y. Lai, Z.A. Zhang, Y. Liu, G. Zhang, H. Huang, Polyaniline nanowire array encapsulated in titania nanotubes as a superior electrode for supercapacitors, Nanoscale 3 (2011) 2202, https://doi.org/10.1039/c0nr00899k. [2] M. Kim, I. Oh, J. Kim, Influence of surface oxygen functional group on the electrochemical behavior of porous silicon carbide based supercapacitor electrode, Electrochim. Acta 196 (2016) 357–368, https://doi.org/10.1016/j. electacta.2016.03.021. [3] J. Wang, X. Zhang, Q. Wei, H. Lv, Y. Tian, Z. Tong, X. Liu, J. Hao, H. Qu, J. Zhao, Y. Li, L. Mai, 3D self-supported nanopine forest-like Co3O4@CoMoO4 core–shell architectures for high-energy solid state supercapacitors, Nano Energy 19 (2016) 222–233, https://doi.org/10.1016/j.nanoen.2015.10.036. [4] G. Zhang, T. Wang, X. Yu, H. Zhang, H. Duan, B. Lu, Nanoforest of hierarchical Co3O4@NiCo2O4 nanowire arrays for high-performance supercapacitors, Nano Energy 2 (2013) 586–594, https://doi.org/10.1016/j.nanoen.2013.07.008. [5] M. Sevilla, R. Mokaya, Energy storage applications of activated carbons: supercapacitors and hydrogen storage, Energy Environ. Sci. 7 (2014) 1250–1280, https://doi.org/10.1039/C3EE43525C. [6] M. Beidaghi, W. Chen, C. Wang, Electrochemically activated carbon microelectrode arrays for electrochemical micro-capacitors, J. Power Sources 196 (2011) 2403–2409, https://doi.org/10.1016/j.jpowsour.2010.09.050. [7] H. Zhang, G. Cao, Z. Wang, Y. yang, Z. Shi, Z. Gu, Growth of manganese oxide nanoflowers on vertically-aligned carbon nanotube Arrays for high-rate electrochemical capacitive energy storage, Nano Lett. 8 (2008) 2664–2668, https://doi.org/10.1021/nl800925j. [8] K. Wang, Q. Meng, Y. Zhang, Z. Wei, M. Miao, High-performance two-ply yarn supercapacitors based on carbon nanotubes and polyaniline nanowire arrays, Adv. Mater. 25 (2013) 1494–1498, https://doi.org/10.1002/adma.201204598. [9] J. Li, G. Zhang, N. Chen, X. Nie, B. Ji, L. Qu, Built structure of ordered vertically aligned codoped carbon nanowire arrays for supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 24840–24845, https://doi.org/10.1021/acsami.7b05365. [10] L.L. Zhang, R. Zhou, X.S. Zhao, Graphene-based materials as supercapacitor electrodes, J. Mater. Chem. 20 (2010) 5983–5992, https://doi.org/10.1039/ C000417K.
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[35] C.J. Lee, G. Pezzotti, Y. Okui, S. Nishino, Raman microprobe mapping of residual microstresses in 3C-SiC film epitaxial lateral grown on patterned Si(111), Appl. Surf. Sci. 228 (2004) 10–16, https://doi.org/10.1016/j.apsusc.2004.01.052. [36] Q. Sun, P. Zhu, Q. Xu, R. Tu, S. Zhang, J. Shi, H. Li, L. Zhang, T. Goto, J. Yan, S. Li, High-speed heteroepitaxial growth of 3C-SiC (111) thick films on Si (110) by laser chemical vapor deposition, J. Am. Ceram. Soc. 101 (2018) 1048–1057, https://doi. org/10.1111/jace.15260. [37] P. Wang, T. Hayashi, Q. Meng, Q. Wang, H. Liu, K. Hashimoto, L. Jiang, Highly boosted oxygen reduction reaction activity by tuning the underwater wetting state of the superhydrophobic electrode, Small 13 (2017) 1601250, https://doi.org/ 10.1002/smll.201601250. [38] K. Hernandez Ruiz, J. Liu, R. Tu, M. Li, S. Zhang, J.R. Vargas Garcia, S. Mu, H. Li, T. Goto, L. Zhang, Effect of microstructure on HER catalytic properties of MoS2 vertically standing nanosheets, J. Alloy. Comp. 747 (2018) 100–108, https://doi. org/10.1016/j.jallcom.2018.02.347.
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Nanoforest of 3C–SiC/graphene by laser chemical vapor deposition with high electrochemical performance Qingyun Sun a, Rong Tu a, Qingfang Xu a, Chitengfei Zhang a, Jun Li b, Hitoshi Ohmori c, Marina Kosinova d, Bikramjit Basu e, Jiasheng Yan f, Shusen Li f, Takashi Goto a, Lianmeng Zhang a, Song Zhang a, * a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, China National Key Laboratory for Shock Wave and Detonation Physics, Institute of Fluid Physics, P.O. Box 919-102, Minyan, 621900, China Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan d Nikolaev Institute of Inorganic Chemistry, Russian Academy of Sciences Siberian Branch, 3 Acad. Lavrerntiev Pr. Novosibirsk, 630090, Russia e Centre for BioSystems Science and Engineering, Indian Institute of Science, Bangalore, 560012, Karnataka, India f Tech Semiconductors, LTD, No.162 Shengli Street, Xiangyan, 441021, China b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Nanoforest of 3C–SiC/graphene was deposited by laser chemical vapor deposition. � The material shows capacitance of 8.533 mF/cm2 at a current density of 20 μA/cm2. � Improved capacitances per area and excellent cycle stability were observed.
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoforest 3C–SiC films Graphene LCVD Electrochemical performance
Nanoforests of 3C–SiC/graphene films are prepared by laser chemical vapor deposition (LCVD). The effectiveness of nanoforest-like 3C–SiC/graphene film as electrode materials for supercapacitors has been investigated by cyclic voltammetry and galvanostatic charge-discharge tests in 0.5 M H2SO4 solution. The specific capacitance is 8.533 mF/cm2 at a current density of 20 μA/cm2, which is 15 times higher than of previous reports of composited 3C–SiC/graphene films. The electrode exhibits good rate capability and cycling stability with 90.5% capacitance retention after 10000 cycles. The nanoforest-like 3C–SiC/graphene thick film shows a 3D porous structure with exposed graphene conductive network contributing to the greatly enhanced electrochemical performance in an environmentally aqueous electrolyte. These nanoforest-like 3C–SiC/graphene films can be promising for elec trochemical energy storage applications.
* Corresponding author. E-mail address: [email protected] (S. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227308 Received 19 July 2019; Received in revised form 13 September 2019; Accepted 13 October 2019 Available online 22 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
electrode has been investigated in detail and further compared with previous researches.
The ever-increasing pollution concerns and energy demands have kindled worldwide research interest in developing more efficient and clean alternative energy storage devices [1]. Supercapacitors, especially electrical double-layer capacitors (EDLCs) have attracted more and more attention due to their significant advantages, such as high power density, long cycle-life, and safe operation with high rate charge and discharge [2–4]. For planer micro-supercapacitor, both the electrode properties and large-scale manufacturability need to be considered. Carbon-based materials, including activated carbon [5,6], carbon nanotubes [7,8], carbon nanowires [9] and graphene [10] have been widely investigated as electrode materials in supercapacitor. Among them, graphene has attracted much attention due to higher specific surface area and high conductivity [11–13]. However, in most studies, graphene capacitors electrodes are fabricated by organic binders from powders. The graphene layers intend to restack, leading to reduced accessible surface area with electrolyte [14,15]. Recently, 3C–SiC materials have been demonstrated recently as a promising semiconductor material for planer micro-supercapacitors due to the excellent chemical stability, good mechanical robustness, wide band gap and high electron mobility. Especially, 3C–SiC can be depos ited by chemical vapor deposition below Si melting point (1683 K), which can be compatible with the current Si based semiconductor in dustry [16]. Several efforts focused on 3C–SiC planer micro-supercapacitor electrodes (on Si chips) have already been re ported in the literature [2,17–23]. Yang et al. have demonstrated the potential of nanocrystalline 3C–SiC films to be used as supercapacitor electrodes [24,25]. Although it has a high surface efficiency, due to a low surface area of nanocrystalline film structure, its specific capaci tance was only 0.073 mF/cm2. Alper et al. have fabricated 3C–SiC nanowires on Si substrate by low pressure CVD method. The electrodes achieved capacitance values to 0.240 mF/cm2 [17]. However, 3C–SiC materials exhibit low electrical conductivity which limits its perfor mance as EDLCs electrodes [14]. The performance of EDLCs is greatly depended on the specific surface area as well as the conductivity and stability of their electrode materials. The excellent electrical conductivity of the electrode materials can significantly reduce the internal resistance of an electrode, by forming a conductive network, and facilitate the application of electrostatic charges, which favor the accumulation of electric double layers [2,26]. The addition of graphene as filler was proved to significantly enhance the electrical conductivity of 3C–SiC materials [27,28]. However, a few studies were reported to use the 3C–SiC/graphene as electrode materials for supercapacitors. Currently, Heuser at el [14] synthesized 3C–SiC/graphene hybrid nanolaminate and investigated the perfor mance as supercapacitors electrode. The 3C–SiC/graphene electrode exhibited a high specific capacitance of 0.550 mF/cm2 which are significantly higher than uniform 3C–SiC films [24]. The layer number of graphene was about 15 between 3C–SiC layers. However, the elec trical activity and wettability of graphene will dramatically reduce by numbers of stacking layers, which will limit its performance in super capacitors [17,29]. Furthermore, the distribution and exposed area of graphene surface (active area) plays important role on supercapacitors performance. Hence, there is a need for controlling 3C–SiC/graphene film morphology with higher surface area directly on Si chips that can keep stable excellent capacitive behavior for long time circle and adapt to microfabrication process in manufacturing of planar micro-supercapacitor devices. Laser chemical vapor deposition (LCVD) was developed by our research group which could control different structure and morphology of films [16,30]. By controlling the morphology of porous and conduc tive electrode materials is an effective way to provide them with high surface areas and efficient paths for electron diffusion [2]. In this study, we have carried out 3C–SiC/graphene nanoforest films directly on Si substrate. The performance of 3C–SiC/graphene nanoforest as EDLCs
2. Experimental 2.1. Preparation of 3C–SiC/graphene films A self-made horizontal cold-wall laser chemical vapor deposition (LCVD) apparatus was employed to deposit nanoforest-like 3C–SiC films on Si (110) single crystalline substrate (15 � 10 � 0.5 mm, Hefei Kejing Inc., China), with hexamethyldisilane (HMDS, Tokyo Chemical Inc., Japan) as a single source. H2 (purity 99.999%, Wuhan Minghui Inc., China) and Ar (purity 99.999%, Wuhan Minghui Inc., China) was used as dilution gas and carrier gas, respectively. The laser (HK1064–500FC, wavelength: 1064 nm, Beijing ZK laser, China) was operated in the continuous mode. The deposition temperature (Tdep) ranged from 1473 to 1623 K and was measured by an infrared pyrometer (CT laser, 673–1873 K, Optris, Germany). The total pressure and deposition time was at 400–1200 Pa and 20 min, respectively. The source HMDS was evaporated from a stock tank at 300 K and was carried into the depo sition chamber with Ar and H2 gas. The flow rate of diluted H2 gas and carrier Ar gas was set as 500 and 25 standard cubic centimeter per minute (sccm), respectively. Fig. 1 shows the schematic of the LCVD apparatus and the synthesis process of nanoforest of 3C–SiC/graphene. In the LCVD process, the morphology of film can be designed by adjusting the deposition parameters to control the nucleation rate and growth rate of 3C–SiC. In order to obtain the higher specific surface, the nanoforest-like 3C–SiC overlapped by nano-whiskers was deposited at 1523 K and 400 Pa. The graphene and 3C–SiC growth with one step. In the deposition process, the laser and the H2 has an etching effect on SiC whiskers, the Si atom on the edge will be etched by H2 forming Si atom vacancy. Then the rest C atoms combined each other by C–C band to grow graphene epitaxially on SiC whisker edge. 2.2. Structure characterization The surface and cross-section morphology of deposited films were observed by field emission scanning electron microscopy (FESEM; Quanta-250, FEI, Houston, USA; 20 kV). Raman spectrum (Invia, Renishaw, UK; 514.5 nm) measurement was used to evaluate the composition and phase of deposited films. Scan Transmission electron microscopy (STEM, TALOS F200S, USA) was used to observe the 3D morphology of the nanoforest-like 3C–SiC/graphene film obviously. The Energy Dispersion Spectroscopy (EDS) on STEM was used to analysis the element and distribution of nanoforest-like 3C–SiC/graphene film. Transmission electron microscopy (TEM, JEOL JEM-2100, Tokyo, Japan; 200 kV) was used to observe the micro-structure and phase of the deposited films. 2.3. Electrochemical and wettability testing Capacitive performance of the films was investigated using an elec trochemical workstation (CHI 660e) in a three electrodes system. The electrochemical properties were measured by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) methods. A Pt foil was used as counter electrode, and a Saturated Calomel Electrode (SCE) was used as the reference electrode. All the measurements were performed in an aqueous electrolyte solution of 0.5 M H2SO4 at room temperature. Prior to acquisition, samples were cycled 20 times to ensure removal of any adsorbed contaminants. The wettability of 0.5 M H2SO4 electrolyte solution on surface of films was conducted on contact angle measurement system. 3. Results and discussion Fig. 2 shows the typical surface and cross-section SEM micrograph of 2
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Fig. 1. The schematic of the LCVD apparatus and the synthesis process of nanoforest of 3C-SiC/graphene film
different morphologies of deposited 3C–SiC films. Fig. 2 (a) presents the surface image of nanoforest-like 3C–SiC film with magnified surface image of single whisker showed in the right-corner. A high density of 3C–SiC nano-whiskers with average diameters about 60 nm and length about 500 nm were grown on the surface. The whiskers looked like needles with sharp tips. Fig. 2 (d) shows the cross-section image of the nanoforest-like 3C–SiC film, and the magnified image of the crosssection growth was shown in Fig. 2 (g). The numerous SiC nanowhiskers like the branches grew around an imaging trunk with the top oblique upward, appearing like “nanoforest” standing vertically on the Si substrate. The surface and cross-section images of the flower-like and pyramid-like films were shown in Figs. 2 (b), (e), (h) and Figs. 2 (c), (f), (i), respectively. The typical grain morphology of the flower-like and pyramid-like film was shown in the right-corner of Figs. 2 (b) and (c), respectively. Some intervals existed between grains of nanoforest-like and flower-like films, as shown in Fig. 2 (g) and (h), while the crosssection image of pyramid-like film was dense, as shown in Fig. 2 (i). The simulated diagrams of three typical morphologies (nanoforest-like, flower-like and pyramid-like) were shown in corresponding areas in Fig. 2 The thickness of nanoforest-like, flower-like and pyramid-like 3C–SiC film was about 20.7 μm, 13.8 μm, 12.1 μm in 20 min deposi tion process by LCVD, respectively. The effect of Tdep and Ptot on the morphologies of films prepared by LCVD was shown in Fig. S1. The nanoforest-like films were obtained at low Tdep. With increasing Tdep, the crystalline of films increased, the grains of film grew densely. At high Tdep and low Ptot, the pyramid-like films were obtained. Whereas at high Tdep and high Ptot, the flower-like films were deposited, this is probably due to a relative higher concen tration of HMDS at higher Ptot [31]. The selected samples represent three typical morphology as marked A, B and C will been compared in later discussions.
Fig. 3 shows the Raman spectra of selected films (A, B, C) at different Tdep and Ptot. The Raman shift from 550 to 1200 cm 1 was conducted in the Raman test. The peak at ~1345 cm 1 and ~1598 cm 1 was related to carbon D and G band, respectively. The observation of carbon 2D band at ~2675 cm 1 indicated the presences of multilayered graphene [32]. The characteristic Raman modes of 3C–SiC are transverse optical (TO) phonon and longitudinal optical (LO) phonon, corresponding to 796 and 972 cm 1, respectively [24]. In the case of sample A (nano forest-like films) deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa and sample B (flower-like films) deposited at Tdep ¼ 1573 K and Ptot ¼ 800 Pa, the typical features for 3C–SiC as well as for graphite and graphene were observed, confirming the formation of 3C–SiC/Graphene composited films [33]. The D’ peak at 2950 cm 1 indicated the defects in graphene [14]. The TO peak was much higher than LO peak which was probably caused by stacking defects in the 3C–SiC film [34,35]. While in the case of sample C (pyramid-like films) deposited at Tdep ¼ 1623 K and Ptot ¼ 400 Pa, TO and LO peaks without characteristic peak for carbon phase were observed, indicating pure phase of 3C–SiC deposits. Fig. 4 shows the TEM observations and atom simulated structure of nanoforest-like 3C–SiC/graphene film deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. The TEM image of cross-section morphologies for nanoforest-like 3C–SiC/graphene film was shown in Fig. 4(a). In the film part, the black area presented the overlapped 3C–SiC nano-whiskers, the pores between whiskers can been vaguely seen from the penetrated light. The selected area electron diffraction (SAED) analysis for the nanoforest-like 3C–SiC film in white square of Fig. 4(a) showed a pattern of three rings (Fig. 4(b)), indicating the film is polycrystalline growth. The polycrystalline rings are indexed to (111), (220), (311) plane of 3C–SiC, respectively. Fig. 4(c) shows the cross-section morphology of the individual 3C–SiC nano-whiskers. Fig. 4(d) shows high-resolution 3
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Fig. 2. Surface and cross-section FESEM images of different morphologies for deposited films. (a), (b), (g): Nanoforest-like film deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. (b), (e), (h): Flower-like film deposited at Tdep ¼ 1573 K and Ptot ¼ 800 Pa. (c), (f), (i): Pyramid-like film deposited at Tdep ¼ 1623 K and Ptot ¼ 400 Pa.
transmission electron microscopy (HRTEM) for the edge of 3C–SiC sin gle whisker in the black square of Fig. 4(c). The lattice fringe spacing of 0.252 nm as heighted is related (111) and (11–1) plane (with a 71� angle) of 3C–SiC, meaning that the 3C–SiC whiskers grew in <111> orientation. The lattice fringe spacing of 0.334 nm as heighted in Fig. 4 (d) is related graphene growth. The growth mechanism of 3C–SiC/gra phene nanoforest was discussed by atomic configuration, as shown in Fig. 4(e). Fig. 4(d) was observed from [ 1-12] axis, while Fig. 4(e) was simulated from Refs. [1–10] axis, since the generated twins of 3C–SiC only can be observed from Refs. [1–10] axis. The twinning boundary (TB) was on (111) plane, as simulated in Fig. 4(e). Under the irradiation of laser in H2 atmosphere, the Si atom on the edge of waved (11–1) plane will be etched by H2, forming Si atom vacancy [36]. Then the rest C atoms combined each other by C–C band to grow graphene epitaxially on the (11–1) plane of SiC whisker edge, as shown in Fig. 4(e). The growth direction of SiC whisker and graphene are vertical. The graphene grew on the SiC (11–1) plane, because a waved (11–1) plane formed by TB growth has a relative lower surface energy. Fig. 5 shows the STEM image and element distribution analysis for nanoforest-like 3C–SiC/graphene film deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. The STEM images of cross-section morphologies for nanoforest-like 3C–SiC film were shown in Fig. 5 (a) and (b). In the film
part, the bright area presented the overlapped 3C–SiC nano-whiskers, the pores between whiskers can be seen from the penetrated dark area obviously. The magnified STEM image of the cross-section nanoforestlike 3C–SiC/graphene film was shown in Fig. 5 (b). The growth direction of all the whiskers was random by crisscross overlapped each other, forming a 3D pores network structure. Compared with TEM image, the STEM image is more efficient to characterize the pores network struc ture of nanoforest-like 3C–SiC film. The element analysis of nanoforestlike 3C–SiC/graphene film was characterized by energy dispersion spectroscopy (EDS), as shown in Fig. 5 (c)–(f). Only signals of C and Si are observed in the EDS analysis. The topography consisted of Si element (Fig. 5 (c)) was corresponding to the whole topography of nanoforestlike 3C–SiC/graphene film (Fig. 5 (a)). Some additional C element related to graphene was distributed in the pores area between 3C–SiC whiskers, as shown in Fig. 5 (e). This EDS map also indicated that the graphene was formed on the edge of 3C–SiC whiskers, corresponding the TEM results (Fig. 4(d)). The TEM observations for pyramid-like 3C–SiC film deposited at Tdep ¼ 1623 K and Ptot ¼ 400 Pa was shown in Fig. S2. The TEM image of cross-section morphologies for pyramid-like 3C–SiC film was shown in Fig. S2(a). The grains grew with pyramid tips, related to SEM image (Fig. 2(f)). A polycrystalline diffraction pattern was also observed in the 4
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selected area electron diffraction (SAED) analysis for the pyramid-like 3C–SiC film (Fig. S2(b)). While some strong diffraction spots including diffraction spot from (111) plane of 3C–SiC in growth direction were obvious, indicating strong <111> 3C–SiC orientation. Fig. S2(c) shows the cross-section morphology of 3C–SiC grains. The grains were larger than 600 nm. Fig. S2(d) shows high-resolution transmission electron microscopy (HRTEM) for the boundary of 3C–SiC grains in the square of Fig. S2(c). There was no graphene found on the boundary of 3C–SiC grains as well as other area of films. Since the CV and GCD measurements were conducted in aqueous electrolyte, the wettability of electrode has a direct influence on the capacitive performance. Fig. S3 shows the wettability of different mor phologies of deposited films. The contact angle is dependent tightly on the surface morphology of the film, especially on the space between the grains on surface [37]. The surface of nanoforest-like 3C–SiC/graphene film had a good hydrophilia with a contact angle of 48� . While the flower-like 3C–SiC/graphene film and pyramid-like 3C–SiC film showed a hydrophobic with contact angle of 105� and 132� , respectively. A small contact angle would result in a better capacitive performance, which will improve the affinity of the electrode towards the electrolyte [38]. The electrochemical properties of the films were characterized using cyclic voltammetry. The double layer capacitances of the films were first evaluated in the potential window of 0.5–0.9 V with scan rate from 10 to 50 mV/s in 0.5 M H2SO4. Fig. 6 (a) show the CV curves at a scan rate of 50 mV/s. For a potential window of 0.4 V, the shape of the CV curves of
Fig. 3. Raman spectra of the films deposited at different Tdep and Ptot. (A): Nanoforest-like films deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. (B): Flower-like films deposited at Tdep ¼ 1573 K and Ptot ¼ 800 Pa. (C): Pyramidlike films deposited at Tdep ¼ 1623 K and Ptot ¼ 400 Pa.
Fig. 4. TEM observations ((a)-(d)) and the atomic configuration (e) of the nanoforest-like 3C-SiC/graphene films deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa. 5
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Fig. 5. STEM morphology observation ((a), (b)) and EDS analysis ((c)-(e)) of the nanoforest-like 3C-SiC/graphene film deposited at Tdep ¼ 1523 K and Ptot ¼ 400 Pa.
three samples were symmetrical and rectangular, indicating an ideal capacitive behavior [39]. The current density of the nanoforest-like and flower-like 3C–SiC/graphene films were much higher than pyramid-like 3C–SiC films. Its double layer capacitances (Cdl, mF/cm2) were calcu lated by using the equation of C ¼ j/ν (j is the background current density in mF/cm2 and ν is the scan rate in V/s) [24]. The average specific capacitances (Cdl) of three samples were calculated at scan rate from 10 to 50 mV/s as shown in Fig. 6 (b). Note that, the plane surface area of the films was taken into consideration to calculate the current density and Cdl, the inside area of film was not considered. The average specific capacitances of pyramid-like 3C–SiC films was only 0.02 mF/cm2. The value is close to epitaxial 3C–SiC films (0.006 mF/cm2) [24]. While for nanoforest-like and flower-like 3C–SiC/graphene films, the Cdl was 7.350 mF/cm2, 2.830 mF/cm2, respectively, which are more than 140–360 times higher compared to that of pyramid-like 3C–SiC film. The significant difference of the capacitance of three morphologies (sample A, B, C) comes from there variant specific surface area and conductivity, since the capacitance of a capacitor electrode is proportional to its active surface area and con ductivity [40]. The graphene formed on the boundary of SiC grains strongly improve the conductivity of deposited 3C–SiC/graphene films in comparison with pure 3C–SiC films [31]. In addition, compared with flower-like 3C–SiC/graphene films, the better hydrophily and porous network structure of nanoforest-like 3C–SiC/graphene film will contribute to the high electrochemical reaction surface in electrolyte, resulting in higher specific capacitance [14]. The nanoforest-like 3C–SiC/graphene films are expected to be a promising candidate for supercapacitor applications, since it shows a relatively higher double layer capacitance. The capacitive behavior of the nanoforest-like 3C–SiC/graphene films with a potential window from 0.2 to 1.0 V has been investigated. Fig. 6 (c) show the CV curves of the nanoforest-like 3C–SiC/graphene film in 0.5 M H2SO4 at scan rate from 10 to 100 mV s. The CV curves for all the scan rates showed quasi-
rectangular shape obviously in the potential range scanned. The CV curve was symmetric, as expected for a typical characteristic of a capacitor. The charge-discharge behaviors at different current density from 20 to 200 μA/cm2 were shown in Fig. 6 (d). For all current densities tested, the segments of anodic charging curves were almost symmetric to the cathodic discharge curves, indicating the perfect reversibility properties of its capacitor behavior. The average specific capacitance was thus calculated from the CV curves (C ¼ j/ν, j is the background current density in mF/cm2 and ν is the scan rate in V/s) and the chargedischarge curves (C ¼ jΔt/ΔV, j is the background current density in mF/ cm2, ΔV is the potential range in V and Δt is the discharge time in s), as shown in Fig. 6(e) and Fig. 6 (f), respectively. In the case of charge current density, the specific capacitance decreased by 36% (from 8.533 to 5.425 mF/cm2 with a charge current density from 20 to 200 μA/cm2). The same as other capacitors, its specific capacitance decreased with an increase of the scan rate and current density. Nevertheless, only 20% reduction of its capacitance was observed even with 20-fold increase of the scan rate (from 8.480 to 6.680 mF/cm2 with a scan rate from 10 to 200 mV/s). With 20-fold increase of the scan rate (with a scan rate from 10 to 200 mV/s), the nanocrystalline 3C–SiC films on Si substrate showed a 34% reduction of its capacitance [24]. While the SiC nano wires on carbon fabric showed a 50% reduction of its capacitance [21]. Therefore, good rate capability has been achieved on the nanoforest-like 3C–SiC/graphene films. Fig. 7 shows the Nyquist spectra of the nanoforest-like 3C–SiC/gra phene films measured from 10 kHz to 0.01 Hz. From the Nyquist plot, solution resistance can be got from the Z’ axis intercept at the high frequency end. Normally, the spectrum shows a semicircle which is represented by a parallel combination of interfacial capacitance and resistance. The width of semicircle plotted was indicative of the chargetransfer resistance (Rct) in the electrode. In the mid-frequency range, the spectrum shows the Warburg diffusion element, an important factor to affect the performance of supercapacitors. At very low frequencies, a 6
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Fig. 6. (a) CV curves for different mor phologies of films at 50 mV/s within a po tential window of 0.4 V and (b) linear fitting of the background current density (j) vs scan rate; (c) CV curves within a potential win dow of 0.8 V for nanoforest-like 3C-SiC/gra phene films at different scan rates; (d) charge-discharge curves for nanoforest-like 3C-SiC/graphene films with different cur rent densities; specific capacitances of the nanoforest-like 3C-SiC/graphene film vs scan rate (e) and charge current density (f).
vertical line can be observed which is caused by the accumulation of ions at the bottom of the pores of the electrode [2,23]. The Nyquist plot is similar to the previous report of composited 3C–SiC/graphene films [14], and the Rct of nanoforest-like 3C–SiC/graphene film was calcu lated from the Nyquist spectra with 126.5 Ω. The cycle durability is one of the most electrochemical properties of supercapacitors. Typical Nyquist plots the nanoforest-like 3C–SiC/graphene electrode after 1st and 10000th cycles are shown in Fig. 7. The arc increment from the 1st to the 10000th cycles is not obvious, indicating the nanostructures are well maintained and preserved overall with little structural deformation after 10000 cycles. Furthermore, the increase of the Warburg resistance after 10000 cycles can be attributed to the loss of adhesion of some active materials blocking the diffusion path ways of electron during the charge–discharge process. The long-time cyclic specific capacitance variation of nanoforest-like
3C–SiC/graphene films by galvanostatic charge-discharge (with poten tial from 0.5 to 0.9 V) at a current density of 100 μA/cm2 for 10000 cycles was presented in Fig. 8 (a). A capacitance loss about 3% is ob tained after about 200 circles. After 5000 cycles, the specific capacitance of nanoforest-like 3C–SiC/graphene electrode changes from 6.350 mF/ cm2 of first cycle to 5.950 mF/cm2, which keeps 93.7% capacitance retention. Whereas the capacitance of 3C–SiC/graphene hybrid nano laminate remained about 86% after 5000 cycles [14]. Similar stability of 87–90% has been reported on SiC/graphene nanoparticle after 6000 cycles [41]. After 10000 cycles, the capacitance was 5.750 mF/cm2, which keeps 90.5% capacitance retention after 10000 cycles. The 10 cycles of charge-discharge curves of the first and after 10000 cycles were presented in Fig. 8 (a). Therefore, good cycle stability has been achieved on the nanoforest-like 3C–SiC/graphene films. The highest specific capacitance of nanoforest-like 3C–SiC/graphene 7
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illustration of 3C–SiC/graphene nanoforest films with stable framework and continuous electron pathways. The outstanding electrochemical properties of nanoforest-like 3C–SiC/graphene film would attribute to three factors. Firstly, the composite of 3C–SiC/graphene electrode can combine the advantage of the excellent chemical stability, good mechanical robustness from 3C–SiC and high conductivity, good electrochemical activity from gra phene. The addition of graphene in 3C–SiC will dramatically improve the conductivity that attribute to significant increased capacitance. Secondly, the nanoforest-like 3C–SiC/graphene film showed a 3D porous network structure with good hydrophily will contribute to the high electrochemical reaction surface in aqueous electrolyte. Both the 3C–SiC whiskers and graphene layers are in nanoscale resulting high specific surface. Third, as schematically demonstrated in Fig. 8 (b), the graphene grown epitaxially on the edge of single 3C–SiC nano-whiskers in the forest structure constituted an electrically conductive network. The graphene in 3D pores provide efficient pathways for electron transport. The nanoforest-like 3C–SiC/graphene film directly grew on Si substrate avoiding the use of binders. Intervals were existed between 3C–SiC/graphene nano-whiskers, and substantially avoided the stacking issues and the “dead volume” of graphene in the electrode. Compared with the previous reports of composited 3C–SiC/graphene films, the layers of graphene were more than 10 layers, Since the electrical activity of graphene will dramatically reduce by numbers of stacking layers, which will limit its performance in supercapacitors. The graphene sur face in present electrode with less layers (2–3 layers) are highly exposed inside of 3C–SiC/graphene nanoforest. Therefore, the high electrical conductivity and high specific capacitance of graphene, together with good cycle ability of 3C–SiC and 3D pores forming in 3C–SiC/graphene nanoforest attributed to significantly improved electrochemical perfor mance due to the synergistic effects.
Fig. 7. Nyquist plots of the nanoforest-like 3C-SiC/graphene electrode. Inset magnifies the spectra in the high-frequency range with first and after 10000 circles.
film reached to 8.533 mF/cm2 at a current density of 20 μA/cm2. The highest volumetric capacitance was 4.1 F/cm3 calculated from the CV curves at 10 mV/s. Table 1 compares the double-layer capacitance of SiC film materials used for double-layer capacitor applications. The specific capacitance of nanoforest-like 3C–SiC/graphene film in present study was much higher than 3C–SiC films on Si substrate [17,24,42]. While this value enhanced 15 times higher than of previous reports of composited 3C–SiC/graphene films [14]. Fig. 8 (b) shows the schematic
Fig. 8. (a) Cycling performance of 3C-SiC/graphene nanoforest films of charge-discharge at 100 A/cm2. (b) Schematic illustration of 3C-SiC/graphene nanoforest films with stable framework and continuous electron pathways. Table 1 Specific capacitance of the SiC film materials used for planar double-layer capacitor applications. Electrode material
Substrate
Electrolyte
Double-layer capacitance (mF/cm2)
Volumetric capacitance (F/cm3)
Year
Refs
3C–SiC/graphene nanoforest films 3C–SiC/graphene hybrid nanolaminate Nanocrystalline 3C–SiC films Epitaxial 3C–SiC film SiC nanowires films N-doped Polycrystalline 3C–SiC film
Si Si Si Si Si Si
0.5 M H2SO4 NaSO4 0.1 M H2SO4 0.1 M H2SO4 3.5 M HCl 1 M H2SO4
5.425–8.533 0.458–0.550 0.040–0.073 0.005–0.006 0.135–0.240 0.743
4.1 1.1 1.0 0.2 0.4 2.9
2019 2018 2015 2015 2013 2011
This work [14] [24] [24] [17] [42]
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4. Conclusion
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In summary, LCVD method was used to prepare nanoforest-like 3C–SiC/graphene thick film using hexamethyldisilane (HMDS) in H2 atmosphere. The electrochemical properties of nanoforest-like 3C–SiC thick film as electrode in 0.5 M H2SO4 solution for supercapacitors were studied. The highest specific capacitance was 8.533 mF/cm2, which are 15 times higher than previous reports. The 3C–SiC/graphene nanoforest electrode shows good rate capability and cycling stability with 90.5% capacitance retention after 10000 cycles. The improved electrochemical properties of the electrodes are attributed to synergistic effects of high electrical conductivity and high specific capacitance of graphene, together with good cycle ability of 3C–SiC and 3D pores forming in 3C–SiC/graphene nanoforest. Therefore, 3C–SiC/graphene nanoforest films are potential electrode materials for big capacitance and highly stable energy storage devices. Acknowledgments This work was supported by the Joint Fund of the Ministry of Edu cation for Pre-research of Equipment, China (6141A02022257), the Science Challenge Project, China (No. TZ2016001), the National Natural Science Foundation of China, China (Nos. 51861145306, 51872212 and 51972244), and the 111 Project, China (B13035). It was also supported by the International Science & Technology Cooperation Program of China, China (2018YFE0103600, 2014DFA53090), the Technological Innovation of Hubei Province, China (2019AAA030), the Fundamental Research Funds for the Central Universities, China (WUT: 2018YS003, 2018YS016, 2019III030, 2019III028), and the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, China (WUT, Grant No. 2019-KF-12). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227308. References [1] K. Xie, J. Li, Y. Lai, Z.A. Zhang, Y. Liu, G. Zhang, H. Huang, Polyaniline nanowire array encapsulated in titania nanotubes as a superior electrode for supercapacitors, Nanoscale 3 (2011) 2202, https://doi.org/10.1039/c0nr00899k. [2] M. Kim, I. Oh, J. Kim, Influence of surface oxygen functional group on the electrochemical behavior of porous silicon carbide based supercapacitor electrode, Electrochim. Acta 196 (2016) 357–368, https://doi.org/10.1016/j. electacta.2016.03.021. [3] J. Wang, X. Zhang, Q. Wei, H. Lv, Y. Tian, Z. Tong, X. Liu, J. Hao, H. Qu, J. Zhao, Y. Li, L. Mai, 3D self-supported nanopine forest-like Co3O4@CoMoO4 core–shell architectures for high-energy solid state supercapacitors, Nano Energy 19 (2016) 222–233, https://doi.org/10.1016/j.nanoen.2015.10.036. [4] G. Zhang, T. Wang, X. Yu, H. Zhang, H. Duan, B. Lu, Nanoforest of hierarchical Co3O4@NiCo2O4 nanowire arrays for high-performance supercapacitors, Nano Energy 2 (2013) 586–594, https://doi.org/10.1016/j.nanoen.2013.07.008. [5] M. Sevilla, R. Mokaya, Energy storage applications of activated carbons: supercapacitors and hydrogen storage, Energy Environ. Sci. 7 (2014) 1250–1280, https://doi.org/10.1039/C3EE43525C. [6] M. Beidaghi, W. Chen, C. Wang, Electrochemically activated carbon microelectrode arrays for electrochemical micro-capacitors, J. Power Sources 196 (2011) 2403–2409, https://doi.org/10.1016/j.jpowsour.2010.09.050. [7] H. Zhang, G. Cao, Z. Wang, Y. yang, Z. Shi, Z. Gu, Growth of manganese oxide nanoflowers on vertically-aligned carbon nanotube Arrays for high-rate electrochemical capacitive energy storage, Nano Lett. 8 (2008) 2664–2668, https://doi.org/10.1021/nl800925j. [8] K. Wang, Q. Meng, Y. Zhang, Z. Wei, M. Miao, High-performance two-ply yarn supercapacitors based on carbon nanotubes and polyaniline nanowire arrays, Adv. Mater. 25 (2013) 1494–1498, https://doi.org/10.1002/adma.201204598. [9] J. Li, G. Zhang, N. Chen, X. Nie, B. Ji, L. Qu, Built structure of ordered vertically aligned codoped carbon nanowire arrays for supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 24840–24845, https://doi.org/10.1021/acsami.7b05365. [10] L.L. Zhang, R. Zhou, X.S. Zhao, Graphene-based materials as supercapacitor electrodes, J. Mater. Chem. 20 (2010) 5983–5992, https://doi.org/10.1039/ C000417K.
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